1. Field of the Invention
The present invention relates to a solid state imager with reduced smear and a method of manufacturing the same.
2. Description of Related Art
In CCD (charge-coupled device) solid state imagers for 1-inch optical system HD (high definition) television receiver, there are proposed CCD solid state imagers having a shunt line structure in order to prevent a propagation delay of a transfer clock applied to a vertical transfer register. In the specification, "shunt line" is referred to as "low resistance interconnection for connecting transfer electrodes of the same phase".
FIGS. 1 and 2 of the accompanying drawings show a conventional FIT (frame interline transfer) type CCD solid state imager having a shunt line structure, in particular, an example of an imaging section thereof.
As illustrated, a CCD solid state imaging device 1 comprises a silicon substrate 2 of a first conductivity type, e.g., N type, a first well region 3 of a second conductivity type, i.e., P type formed on the N-type silicon substrate 2, an N-type impurity diffusion region 4, an N-type transfer channel region 6 forming a vertical transfer register 5 and a P-type channel stopper region 7, all of which are formed within the first P-type well region 3, a P-type positive electric charge accumulation region 8 formed on the N-type impurity diffusion region 4 and a second P-type well region 9 formed beneath the N-type transfer channel region 6.
A PN junction i between the N-type impurity diffusion region 4 and the P-type well region 3 constructs a photosensor section (photoelectric-conversion section) 10. The photosensor section 10 serves as a pixel, and a plurality of photo-sensor sections 10 are arranged in a matrix shape.
A plurality of transfer electrodes 16 (16A, 16B) made of polycrystalline silicon layer of first and second layers are formed on the transfer channel region 6, the channel stopper region 7 forming the vertical transfer register 5 and a read-out gate section 11 through a gate insulating film 15. The transfer channel region 6, the gate insulating film 15 and the transfer electrodes 16 constitute the vertical transfer register 5. The transfer electrodes 16 are extended in the horizontal direction, respectively. The first layer polycrystalline silicon transfer electrode 16A and the second layer polycrystalline silicon transfer electrode 16B are overlapped each other in the area of the adjacent photosensor sections 10.
Shunt line layers 18 made of a first Al layer are formed on the vertical transfer registers 5 through an interlayer insulating layer 17 so as to be extended in the vertical direction. Each shunt line layer 18 is connected to the corresponding transfer electrode 16 through a contact section 19. 4-phase transfer clocks .phi.V.sub.1 to .phi.V.sub.4, for example, are applied to the shunt line layer 18.
In the aforesaid arrangement shown in FIGS. 1 and 2, the Al shunt line layer 18 is served also as an Al photo-shield layer for reducing a smear.
FIG. 3 shows another example of the CCD solid state imaging device in which a smear component can be reduced. In a CCD solid state imaging device 21, as shown in FIG. 3, a photo-shield layer 22 formed of a second Al layer covers the adjacent photosensor sections 10 in the vertical direction through the interlayer insulating layer, in addition to the arrangement shown in FIG. 1.
According to the CCD solid state imaging device 1 in which the first Al layer is served as both the photo-shield layer on each vertical transfer register 5 and the shunt line layer 18, smear light directly introduced into each vertical transfer register 5 could be shielded but light which becomes incident on the adjacent photosensor sections 10 arranged in the vertical direction could not be shielded so that the light which becomes incident between the adjacent photosensor sections 10 arranged in the vertical direction is leaked to the vertical transfer register 5. As a consequence, a smear suppressed ratio could not be made -100 dB or lower.
Further, according to the CCD solid state imaging device 21 in which the adjacent photosensor sections 10 arranged in the vertical direction are covered with the photo-shield layer 22 formed of the second Al layer, the smear compressed ratio thereof is improved to be several decibels, i.e., about 3 dB as compared with the CCD solid state imaging device 1 shown in FIG. 1. However, the insulating layer formed under the Al photo-shield layer 22 is thick so that a satisfactory smear reducing effect cannot be achieved.
As other method of reducing a smear component, there is proposed a structure in which a reflection preventing film made of TiON, for example, is formed on the lower surface of the Al layer serving as the shunt line layer and the photo-shield layer so as to cover the extended portion of the Al layer into the photo-sensor section. With this structure, although a light component incident on the lower surface of the extended portion of the Al layer into the photo-sensor section can be prevented from being repeatedly reflected to become incident on the vertical transfer register, a scanning of the reflection preventing film remains between the pixels. There is then the risk that the adjacent shunt line layers will be short-circuited. Also, this structure cannot be processed without difficulty and cannot yet be realized.
As a further method of reducing a smear component, there is proposed a structure in which the first Al layer forms the photo-shield layer and the second Al layer forms the shunt line layer. It is very difficult to process this structure, and this structure is not commercially available on the market.